The Debian QEMU packages require a bunch of cross compilers for
building firmware which aren't available on all host architectures.
Using --arch-only skips this particular requirement and allows us to
install just the dependencies we need.
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>